Such structures are used as printed circuit boards, in which the metal layer (mostly copper) is provided in accordance with the desired conductive track pattern. The substrate material used is made of, for example, synthetic resin, a composite material such as glass fiber/epoxy, paper/phenolic resin or ceramic. The substrate material is generally flat and plate-shaped, although it may alternatively be formed by a flat or curved form of, for example, the synthetic resin housing of a domestic appliance or shaver.
A printed circuit board can be manufactured by providing a substrate with a copper foil via an adhesive layer. The copper foil is provided with a photoresist layer and, in succession, exposed to patterned radiation and developed, whereafter the undesired copper is removed in a chemical etching bath. For reasons relating to the protection of the environment, this method is less desirable. In accordance with another method, the substrate, which is non-conductive, is provided with a copper layer in an electroless process. To this end, the substrate is first activated with a noble metal, such as palladium, and then immersed in an electroless metallization bath. Such a bath contains an aqueous solution of, inter alia, a copper salt, a complexing agent such as EDTA, and a reducing agent such as formaldehyde. The copper layer provided in an electroless process is generally reinforced with a copper layer applied by means of electrodeposition. In this process an external current source is used to electrodeposit copper onto the copper layer forming the cathode from an aqueous copper-salt solution.
However, electroless metallization has the drawback that the activation is non-selective, so that the deposited uniform copper layer must subsequently be provided with a pattern in a photolithographic process and by means of chemical etching, which is very laborious. Another drawback is that expensive noble metals (such as palladium) are necessary to activate the substrate surface and that the metallization baths used readily become instable. Moreover, said metallization baths contain toxic components such as the above-mentioned complexing and reducing agents.
A laminated structure of the type mentioned in the opening paragraph is known from an article by S. Gottesgeld et all. in J. Electrochem. Soc. Vol. 139, No. 1, L14 (1992). The known structure is manufactured by providing a substrate with an electrically conductive polypyrrole layer. To this end the substrate is immersed in an aqueous solution of pyrrole and a ferri-salt. After an immersion time in this solution of at least 0.5 hour an electrically conductive polypyrrole layer having a specific conductivity of 60 S/cm is formed on the substrate. The sheet resistance is a few hundred .OMEGA./square. Although in general polymers are poor conductors, it is known that polymers comprising poly-conjugated chains have electrically conductive properties. Said conductivity is referred to as intrinsic conductivity. Examples of such polymers are: polyacetylene, polyaniline, polythiophene and polypyrrole. The conductivity of these polymers is generally low but it can be increased by, for example, (electro)chemically oxidizing, referred to as doping, the polymer, resulting in the formation of p-type conductors. Suitable dopants for obtaining p-type conduction are, for example, I.sub.2, AsF.sub.5, SbF.sub.5, HBF.sub. 4, perchlorates, sulphonates, SO.sub.3 and FeCl.sub.3. This doping results in the formation of positive charge carriers on the polymer chains, which charges are compensated for by negatively charged counter-ions. The sheet resistance of the doped polypyrrole layer thus obtained is sufficiently low to provide this layer with a copper layer in an electrodeposition process.
A disadvantage of the known method is that the copper layer is nonselectively provided, so that it is still necessary to structure the uniform copper layer at a later stage to obtain the desired pattern. According to said article, the copper layer can be structured by removing the undesired polypyrrole from the substrate. The article does not indicate how this could be done; in any case, removal of undesired, conductive polypyrrole requires an additional process step.